Color removal from the effluent streams of paper mills continues to be a problem within the pulp and paper industry. It is necessary that these downstream wastewaters be treated for color removal prior to discharge into public waterways.
The United States wood pulp production capacity is approximately 60 million tons per year. Since the average cellulose content of wood is about 40%, 150 million tons of wood are needed to produce this 60 million tons of pulp. The difference between these two numbers represents the lignin and hemicellulose which must be removed or separated in the pulping process in order to free the cellulose fibers.
The pulping process, however, does not remove 100% of the lignin present in the wood, with approximately 5% remaining after either kraft or sulfite pulping (for mechanical pulping the amount is considerably higher). If a high grade paper is the desired end product, this 5% residual lignin must be removed, and is accomplished by bleaching the pulp.
Since over 35% of the pulp produced in the United States is bleached, there are about one million tons of lignin removed each year at the bleach plant, and most of this in the caustic extraction stage. This number is significant because in the removal process (i.e., bleaching), most of this residual lignin is solubilized. This solubilized lignin is a strong absorber of visible radiation resulting from the conjugation of unsaturated and quinoidal moieties formed during the oxidation step in the bleach plant. Consequently, the bleach plant effluent is highly colored. Although there are other sources of color in paper mill waste effluent, it is readily apparent that where bleaching is performed its effluent can be expected to be the major contributor of waste color. Indeed, at kraft, bleach mills the effluent from the first caustic extraction stage accounts for at least 70% of the waste color.
The goal of the pulping and bleaching operations is the removal of lignin and hemicellulose from the cellulose fiber in the wood. The 95% that is removed by pulping is often burned as fuel in the process of recovering the inorganic chemicals present in the black liquor. In the bleaching operation, the 5% residual lignin is separated from the fibers by degradation and solubilization and ends up in the wastewater. Chemical removal can, therefore, only be accomplished by reducing this solubility, which has proved to be a difficult task.
The process of color removal from the effluent stream is further complicated by the presence of lime, solid particulate matter like pulp, clay, dispersants/surface active materials and polymers used during various stages in the papermaking process. The solid particulate matter is commonly referred to as anionic trash.
Most governmental regulations pertaining to color removal from the effluent stream of a papermaking process are directed to true color, i.e., defined by the EPA/NCASI test as the absorbance of 465 mm of light by a sample adjusted to a pH of 7.6 and filtered through a 0.8 micrometer filter paper. Color is reported in standard color units (scu) which represents the concentration of a color standard solution producing an equivalent degree of absorbance (1 scu=1 mg/l platinum as chloroplatinate).
Nevertheless, there is increasing pressure on pulp and paper mills to lower the apparent color of the effluent water because that is the color visible to the naked eye as the effluent flows into public waterways. Apparent color is unfiltered and not pH adjusted, and results in part from particles that scatter light. There are occasions when the true color of a system that has undergone treatment is low, but the corresponding apparent color is high. This problem is commonly caused by the presence of suspended particulate matter that causes an increase in the turbidity of the system. Therefore, it is important that any new treatment for color removal should not only remove the true color of the effluent, but should also lower the apparent color as well.
The pressure to remove color comes primarily from state environmental agencies. Previously, it was thought that the discharge of colored waste affected only the aesthetic value of the receiving body of water; however, biologists are becoming increasingly concerned about possible toxic effects, the effect of reduced light transmittance through the water causing reduced levels of photosynthetic activity, and, of course, the resultant drop in dissolved oxygen concentration because of this drop in activity. Furthermore, although these colored, waste products are fairly refractory towards biological oxidation and as such the majority of these colored materials are not removed by biological waste treatment plants.
It has been shown that by-products are water soluble, and that a significant amount is produced. This puts severe demands on chemicals to be used for color removal. There are techniques already available, however, that can remove greater than 90% of the color before, within or after biological waste treatment and from isolated waste streams, such as from the alkaline extraction stage of the bleach plant. These techniques include chemical (e.g., alum, ferric, lime or polyelectrolytes), biological (e.g., white rot fungus) and physical processes (e.g., ultrafiltration, ion exchange and carbon absorption). None enjoys widespread use because of unfavorable economics.
The demands on a product used in a color removal application are quite severe, i.e., the product must be capable of reacting with the color bodies in a manner which results in their becoming insoluble and, because of the extremely large amounts of wastewater produced that contains color bodies, the color removal product must work at very low dosage or its use will be precluded by prohibitive costs.
A common problem associated with conventional chemical treatment methods, such as polymers made from epichlorohydrin-dimethylamine (Epi-DMA), is the fact that those polymers cannot lower the color of a system below a certain value beyond which they tend to re-disperse that color. This problem is commonly referred to as "overdosage".
There is therefore a great demand for a product or process that would greatly refine color, both true and apparent from paper mill wastewaters.